SyncMySubsFix subtitle framerate drift
When subtitles start in sync but slowly slide out of time, the video and subtitles were made for different framerates. Rescale the timing to match, instantly and in your browser.
Why framerate mismatches cause drift
Film is usually shot at 23.976 or 24 frames per second. For broadcast and older PAL releases it gets sped up to 25 fps, and for NTSC it may run at 29.97 fps. When subtitles were timed against one framerate and you play them against another, every timestamp is off by a constant percentage rather than a fixed number of seconds. The error is tiny at the start and grows steadily, so by the end of a feature the subtitles can be many seconds out.
The fix is to multiply every timestamp by the ratio of the two framerates. Set From to the framerate the subtitles were built for and To to your video's framerate. If you're not sure, try a preset and watch the preview. A pure offset problem, where the subtitles are off by the same amount throughout, belongs in the Shift tool instead.
FAQ
How do I know it's a framerate problem and not just an offset?
If the subtitles line up at the start but drift further off as the video plays, a little late after ten minutes and badly late by the end, that growing error points to a framerate mismatch. If they are off by the same amount the whole way through, it is a fixed offset, and the Shift tool is the right fix.
Which framerates should I pick?
Set From to the framerate the subtitles were made for and To to your video's framerate. The most common case is film converted for PAL regions: 23.976 fps subtitles played against 25 fps video, or the reverse. If you are unsure, try the 23.976 to 25 and 23.976 to 29.97 presets and preview which one lines up.
Does this re-time every subtitle proportionally?
Yes. It multiplies every timestamp by the ratio of the two framerates, so cues early in the file move a little and cues late in the file move a lot. That cancels out drift that builds up over the length of the video.